The output parameters of many prior art cardiac pacemakers are either preset during fabrication or established at the time of implant. With such pacemakers, adjustment of any output parameter requires a surgical exposure of the pacemaker itself. Other pacemakers are adjustable through the use of a needle-like tool.
More recently, various systems have been advanced for altering the output parameters of an implanted cardiac pacemaker with transmitted signals of electomagnetic energy. The pacemakers of these systems have included circuitry responsive to a preselected signal for altering at least one output parameter of the pacemaker on the occurrence of the signal. For example, in U.S. Pat. No. 3,311,111, the use of bistable magnetic reed switches is proposed for the control of pulse rate, voltage, current or duration as well as the selection of alternate output paths or leads. Other systems have been proposed in which pulse signals are used to advance a counter with the accumulated count in the counter serving to establish the value of the output parameter or parameters to be altered.
Both of the systems described above are susceptible to transient magnetic fields and/or electrical noise. Indeed, a variation of the "pulsed signal" system which reduces probability of output parameter alteration by extraneous noise is advanced in U.S. Pat. No. 3,805,796. In this system, a first counter advances in response to all detected signals while a second counter advances only in response to signals detected after the count of the first counter reaches a preselected value. The value of the count in the second counter is employed to control at least one alterable output parameter. Thus, extraneous signals which are incapable of advancing the first counter to the preselected value cannot result in an alteration of the pacemaker output parameters. However, signals which are capable of advancing the first counter have the inherent ability of advancing the second counter, subject only to their continued presence. Thus, while the system of U.S. Pat. No. 3,805,796 does reduce the probability that a detected extraneous signal will result in an alteration in the pacemaker's output parameters, it does not limit the extraneous signals that will be detected. In essence, this system limits its response to detected signals but, does not limit the signals which it will detect.